error if we don't run any tests

This makes subunit-trace error out if it finds that there were no test
results in the subunit stream. This guards against the case where
something breaks the test toolchain and we stop running tests
entirely. Copied over from equivalent code in Tempest where this has
served us well.

Change-Id: I765395335a68dfc7eb754317f8c45c3dfdd85949